The invention relates to a plasma nozzle for the treatment of surfaces, especially for the pre-treatment of plastic surfaces, with a tubular, electrically conductive housing, which forms a nozzle channel, through which the working gas is flowing, and with a high-frequency generator for applying a voltage between the electrode and the housing.
A plasma nozzle of this type is described in the DE 195 32 412 A1 and is used, for example, for the pretreatment of plastic surfaces, so that the application of adhesives, printing inks and the like on the plastic surface becomes possible or is facilitated. Such a pretreatment is necessary, since, in the normal state, plastic surfaces cannot be wetted with liquids and therefore do not accept the printing ink or the adhesive. The surface structure of the plastic is changed by the treatment, so that the surface can be wetted by liquids with a relatively high surface tension. The surface tension of the liquids with which the surface can still be wetted, represents a measure of the quality of the pretreatment.
By means of the known plasma nozzle, a relatively cool, yet highly reactive plasma jet is achieved, which has the approximate configuration and dimensions of the flame of a candle and therefore also permits the pretreatment of profiled parts with a relatively deep relief Because of the high reactivity of the plasma jet, a very brief pretreatment is sufficient, so that the workpiece can be passed by the plasma jet at a correspondingly high speed. Because of the relatively low temperature of the plasma jet, the pretreatment of heat-sensitive plastics is also possible. Since a counter-electrode is not required on the back of the workpiece, surfaces of block-like workpieces of any thickness, hollow bodies and the like can be pretreated without any problems. For a uniform treatment of larger surfaces, a battery of several plasma nozzles, disposed offset, has been proposed in the aforementioned publication. In this case, however, a relatively large expenditure for equipment is required.
It is therefore an object of the invention to create a plasma nozzle which, in spite of its very compact construction, enables large surfaces of a workpiece to be treated.
This object it is accomplished in the case of a plasma nozzle of the type named above owing to the fact that the outlet of the nozzle channel is constructed as a narrow slot extending transversely to the longitudinal axis of the nozzle channel.
Surprisingly, it has turned out that, by using such an outlet slot, the geometry of the plasma jet can be changed effectively. The plasma jet no longer is in the shape of the flame of a candle and, instead, experiences an extreme expansion within the slot, so that a two-dimensional, yet nevertheless uniform plasma treatment of the workpiece surface becomes possible. If an extended workpiece surface is in front of the opening of the plasma nozzle, the plasma flows to the outside at the diverging edges of the fan and a reduced pressure is developed in the interior of the fan with the result, that the fan-shaped plasma jet is literally xe2x80x9cdrawn inxe2x80x9d to the workpiece, so that the surface of the workpiece comes into intimate contact with the reactive plasma and, accordingly, a very effective surface treatment is achieved.
As in the case of the conventional plasma nozzle, the working gas can be twisted in the nozzle channel. The twisted plasma jet can also be expanded fan-fashion with the help of the outlet slot. At most, the twisting leads to a slight S-shaped distortion of the fan, when looking frontally at the opening of the plasma nozzle.
The intensity distribution of the plasma over the length of the slot can be controlled, for example, owing to the fact that the width of the slot varies over the length. In a preferred embodiment, however, a transverse channel, extending parallel to this slot and having a larger cross-section, in which the plasma can be distributed before it enters the actual outlet slot, is disposed directly upstream of the transverse slot. This arrangement can be produced particularly easily, if the outlet of the nozzle channel, including the slot and the transverse channel, is formed by a separate mouthpiece from an insulating material (ceramic) or preferably from metal, which is pressed or screwed into the housing.
Preferably, the transverse channel is open at either end and these open ends are surrounded only with a certain clearance by the walls of the housing, so that a portion of the plasma can emerge at the ends of the transverse channel and then be deflected by the walls of the housing obliquely in the direction of the workpiece. The plasma fan is then bounded at either edge by particularly intensive edge jets, which literally pull the fan apart. By these means, the shape of the fan and the intensity distribution of the plasma jet within the fan can be adjusted, for example, so that the downstream edge of the plasma fan assumes a concave shape, so that the fan simulates a dovetail. This is particularly advantageous for the pretreatment of convexly curved workpieces, such as cylindrical workpieces, but also proves to be of advantage for the pretreatment of flat workpieces, because the larger distance, which the plasma must cover in the edge regions of the fan before reaching the workpiece, is compensated for by a correspondingly greater intensity of the plasma jet. The contour of the face can be varied by varying the depth, at which the open ends of the transverse channel are retracted in the housing of the plasma nozzle, so that, for example and if necessary, a convex curvature of the downstream edge of the fan can also be reached.
In order to bundle the fan more in the direction perpendicular to the plane of the fan, auxiliary air can be supplied at the outer casing of the housing of the plasma nozzle on both sides of the plane of the fan. In this case, it may be appropriate if the outer surface of the housing of the plasma nozzle is constructed prism-shaped in the opening region and not conically, so that two flat surfaces are formed, which converge towards the plane of the fan.